This invention relates to fuel assemblies for nuclear reactors and, more particularly, to a guide tube arrangement which restrains spacer grid movement and which offers secondary alternative means for supporting a fuel assembly during handling operations.
In water cooled heterogenous nuclear reactors, the reactor core in which the fission chain is sustained generally contains an array of fuel assemblies which are identical in mechanical construction and mechanically interchangeable in any core location. The fuel assemblies are designed to maintain structural adequacy and reliability during core operation, handling, and shipping. Fuel assembly design for core operation typically considers the combined effects of flow induced vibration, temperature gradients, and seismic disturbances under both steady state and transient conditions.
Each fuel assembly contains thin elongated fuel elements, a number of spacer grids, guide tubes, an instrumentation tube, and end fittings. The fuel elements, typically known as fuel rods or pins, house the nuclear fuel. The ends of the fuel elements are sealed with end caps. The fuel elements, guide tubes and instrument tube are supported in a square array at intervals along their lengths by spacer grids which maintain the lateral spacing between these components. The guide tubes are rigidly attached at their extremities to the end fittings. Use of similar material in the guide tubes and fuel elements results in minimum differential thermal expansion. The spacer grids are constructed from rectangular strips or plates which are slotted and fitted together in an "egg crate" fashion. The walls of the square cells, formed by the interlaced strips, contain integrally punched projections which provide lateral support for the fuel elements. Spacer grid to fuel assembly component contact loads are established to minimize fretting, but to allow axial relative motion resulting from fuel element irradiation growth and differential thermal expansion.
Depending upon the position of the assembly within the reactor core, the guide tubes are used to provide continuous sheath guidance for control rods, axial power shaping rods, burnable poison rods, or orifice rods. Clearance is provided within the guide tubes to permit coolant flow therethrough to limit the operating temperature of the absorber materials. In addition, this clearance is designed to permit rod motion within the guide tubes as required during reactor operation under all conditions including seismic disturbances. Joined to each end of the guide tubes are flanged and threaded sleeves which secure the guide tubes to each end fitting by lock welded nuts.
Each fuel assembly is typically installed vertically, in a reactor pressure vessel, on and supported by a core grid assembly support plate. The lower end fitting positions the fuel assembly relative to the core grid plate. The lower ends of the fuel elements rest on the grid of the lower end fitting. Penetrations in the lower end fitting are provided for attaching the lower ends of the guide tubes thereto. The upper end fitting provides means for coupling fuel assembly handling equipment and positioning the fuel assembly within the reactor core.
In operation, the fuel elements in the reactor core become depleted at different rates, those in the center usually being subjected to a higher neutron flux and thus becoming depleted before those near the outside of the core where a lower neutron flux prevails. Consequently, all of the fuel assemblies are not normally replaced at one time but rather in stages. Furthermore, at each refueling, partially depleted elements may be relocated in order to optimize core performance and extend the time between refueling outages.
Generally, spent and new fuel assemblies are transferred from and to the core, respectively, and partially spent fuel assemblies are relocated within the core, by hoists equipped with fuel assembly grapple mechanisms which mechanically engage the upper end fitting. During handling of a fuel assembly, the assembly load, which normally bears on the grid of the lower end fitting, is transferred to the guide tubes. Should a failure of all the giide tubes occur, the fuel assembly's integrity will be destroyed and it will separate into its component parts which may result in damage to the fuel elements and the development of hazardous conditions.
Reactor coolant, under operating conditions, flows relatively parallel to the longitudinal axes of the fuel elements thereby subjecting the leading lengthwise edge of the spacer grid plates to hydraulic forces. Hence, the spacer grid may be subjected to vertical movements or perturbations under steady state or transient conditions or both. In the past, in order to restrain spacer grid movement, sleeves have been attached to the spacer grids and mechanically engaged to protrusions formed on the guide tubes. Alternatively, it has been proposed to engage tab extensions formed on the spacer grid plates with clips attached to the guide tubes in order to restrain spacer grid movement. The use of sleeves, tabs and clips, however, results in the addition of material within the reactor core which is capable of parasitically absorbing neutrons, thereby decreasing reactor efficiency. Moreover, the prior art arrangement required the forming of the protrusions after the guide tube was inserted into the spacer grid complicating both assembly and disassembly procedures. Additionally, use of guide tube clips adds the additional step of welding extraneous material to the guide tube which is capable of disengaging from the tube and being carried through the reactor coolant systems.